Most other DE's and even just regular WM's when used with feh can use a single large wallpaper spanning multiple screens, eg 5760x1080 for triple head. KDE 5 however can not do this and it also blocks feh from working as well. This used to work with KDE 3 and earlier but appears to have broken since then with the rewrite. I wasn't sure which component was the better fit, Image Wallpaper or Multi-screen support so put it under Image Wallpaper since that is likely where it needs to be fixed. https://feh.finalrewind.org/
This is a couple of years old now, but I too would like to see this implemented. Just as an fyi, there is an external program that will accomplish this in the meantime: superpager. It's a little geeky on the interface, but it gets the job done.
*** Bug 436785 has been marked as a duplicate of this bug. ***
A possibly relevant merge request was started @ https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/1800
What if, rather than another Positioning option as proposed in the above MR, a boolean setting was added to span screens? It could be placed at the end of this form layout so it applies to all Positioning options: https://github.com/KDE/plasma-workspace/blob/0fa3bbc4c63e84d5902e0950e4fa0559fe1988cf/wallpapers/image/imagepackage/contents/ui/config.qml#L90 If set to true, make the calculation for the wallpaper based on the dimensions of the entire desktop, rather than the single screen, then crop it to that screen's portion of the desktop based on its x, y, w & h. The exception is the Tiled option where it could simply change the offsets. It looks like the necessary dimensions are exposed: https://doc.qt.io/qt-5/qscreen.html#virtualGeometry-prop and its virtualGeometryChanged signal would allow the wallpaper to be "re-sliced" upon layout changes.
Feel free to submit a merge request. The original author of the MR above is unable to continue the work.